To see classic modern art in Washington, you could head to the curved walls of the Hirshhorn Museum's concrete doughnut. Or to the knife-edged galleries in the East Building of the National Gallery. Or you could take a stroll along 14th Street NW and make a stop at R. There, high on a wall and overlooked by almost everyone, is a work that has all the hallmarks of impressive modernist painting.

At some 40 feet across, our picture has the scale to beat the giant postwar paintings of modern masters such as Jackson Pollock and Barnett Newman. It's also got truly modern colors: A background of the most acid yellow-green, with figures on it in a baby blue, generates a color-clash Picasso would be proud of. Speaking of Picasso, our picture plays almost cubist games with space and perspective: A car hovers in midair, suspended in -- or on? -- the void of that green background, while below it stands a mechanic figure almost out of a futurist painting by Fernand Leger. Or is that worker standing in front of the car? Or simply farther down the picture plane, in a space all of his own? The ambiguities only add to the painting's force.

There are other Picassoid moments.

Under our picture's car, a strange pyramid of empty air is arbitrarily cut out from the scene around it by a heavy black outline, enclosing the mechanic in a disconnected -- even alienated -- world of work. In classic modern form, that outline signals a symbolic separation of the space without worrying too much about any visual reality that such a line might represent. Then, in a moment of almost dada arbitrariness that out-moderns even Picasso, real ivy has been allowed to grow over just this area of the picture, obscuring precisely what a viewer wishes he could see most clearly, while also confusing the painted world in the art work with the real world of the climbing plant. Our picture dissolves the boundary between fiction and reality that more traditional art has often tried to keep intact.

Picasso headed in that direction, too, when he stuck stray scraps of wallpaper or newsprint onto his painted scenes, with no clear reason for their appearance there. That move is echoed in our painting by a random patch of painted checkerboard, almost like a finish-line flag from motor racing, that floats unmoored on the green background.

Our painting also shares Picasso's love of printed symbols and lettering, both in Roman and in foreign scripts. It has a big red arrow, boldly outlined in white then again in black, pointing left along its bottom edge. There's a pile of distorted Chinese characters adrift in the picture's middle. Disjointed English texts scattered across the picture's surface read "A.Y.T. AUTO SERVICE A.Y.T." and "www.auto-ayt.com" and, down at the bottom, "202-797-8800."

Call that number and the phone might very well be answered by Gary Zhu, owner of this and three other A.Y.T. auto-repair shops. Two years ago, he says, he paid $4,000 to have the picture painted on the blank outside wall of his 14th Street garage. He says the eye-catching mural has been very successful in promoting his business and raising its visibility. But he also insists the painting's not a work of art. It's only advertising, painted by a Hispanic customer named Frank whose further particulars he's now mislaid.

With all due respect to Mr. Zhu, he's wrong. Whether he likes it or not, and whether "Frank" intended it or not, the mural is a fine work of modern art.

One of the glories and great innovations of the modern movement was that it wasn't only about the actual objects its artists produced. It was about giving us a whole new perspective on the world around us and the ability to recognize the modern art inherent in it.

That partly came about because the first modern artists borrowed from the surrounding world in the first place. Early last century, for instance, Picasso realized he could steal the look of traditional African masks, just then penetrating Europe, and use them to forge a new kind of European painting. Since then, those masks have always looked strangely like modern art to us, which is really why we've built dedicated art museums to put them in.

Ditto for the distortions found in the art of the untrained and the insane, also admired and copied by modern artists. Such "outsider" art could only count as art to people raised on modernism. Show a madman's coarse drawings to a French academician, circa 1750, and he'd be incapable of seeing the good in them. He'd laugh at a museum dedicated to such art, though we've opened plenty of them over the last 50 years.

But maybe the most important modern moment came in 1913, when Marcel Duchamp insisted he got as much pleasure staring at a revolving bicycle wheel, spinning atop the plain wood stool it had been bolted to, as he did watching a nice log fire. Duchamp eventually coined the term "readymade" for his new kind of art -- anything already in the world that becomes art because you choose to look at it as such, as when a broken lamp shade was brought in from the street by Duchamp's colleague Man Ray and instantly became an elegant modern sculpture called "Lampshade." Yet that was already the true principle behind Picasso's Africanisms, and it went on to be a central principle in much of the art that came later, in every medium. It let Walker Evans find art in photographs of crumbling Southern storefronts; it let Richard Serra make sculpture from almost unchanged industrial sheet steel; it let an innovative video artist such as Douglas Gordon take Hitchcock's "Psycho" and turn it into influential contemporary art, simply by running it 12 times slower than normal.

It's what lets us look at that humble A.Y.T. ad and spot all the modern artness in it -- more, in fact, than we might find in a polished mural that sets out to be art.

Come to think of it, maybe all this isn't that radical or modern of a concept, after all. Classic landscape painters such as Claude Lorrain or John Constable looked at the world around them and chose to paint and recombine the bits that they could turn to artful use. And that made other people, out for walks in the country, find "readymade" Claudes and Constables in the picture-like ("picturesque") nature that they found there.

Finding an accidental work of modern art in our own urban scene may be no stranger than spotting some Constable-like beauty in the untouched English countryside. The fact that the trees don't know they're artful doesn't make them seem any less artistically arranged. Thanks to our Constable-trained eyes, those trees become Constables without even knowing it. Our mural isn't timelessly good art that modern eyes have happened to discover, like gold in river gravel. We couldn't have spotted it -- it couldn't even have existed, as art -- if not for the modern works that came before.